THE MECHANICAL EQUIVALENT OF HEAT. 65 



densation of air as the equivalent of the work done in 

 compressing the air, he obtained a numerical value of the 

 mechanical equivalent of heat. There was, however, in 

 these experiments, one weak point. The matter operated 

 on did not go through a cycle of changes. He assumed 

 that the production of heat was the only effect of the 

 work done in compressing the air. Joule had the merit 

 of being the first to meet this possible source of error. 

 He ascertained that a weight of 1 Ib. would have to fall 

 772 feet in order to raise the temperature of 1 Ib. of 

 water by 1 Fahr. Him subsequently attacked the 

 problem from the other side, and showed that if all the 

 heat passing through a steam engine was turned into 

 work, for every degree Fahr. added to the temperature 

 of a pound of water, enough work could be done to 

 raise a weight of 1 Ib. to a height of 772 feet. The 

 general result is that, though we cannot create energy 

 we may help ourselves to any extent from the great 

 storehouse of nature. Wind and water, the coal-bed 

 and the forest, afford man an inexhaustible supply of 

 available energy. It used to be considered that there 

 was an absolute break between the different states of 

 matter. The continuity of the gaseous, liquid, and 

 solid conditions was first demonstrated by Andrews 

 in 1862. 



Oxygen and nitrogen have been liquefied in- 

 dependently, and at the same time, by Cailletet and 

 Raoul Pictet. Cailletet also succeeded in liquefying air, 

 and soon afterwards hydrogen was liquefied by Pictet 

 under a pressure of 650 atmospheres, and a cold of 170 

 Cent, below zero. It even became partly solidified, and 

 he assures us that it fell on the floor with ' the shrill 



